1. Field of the Invention
The present invention is generally related to a foreign matter inspecting method and a foreign matter inspection apparatus, capable of detecting foreign matters, scratches, defects, contaminations, and the like (these items will be referred to as “foreign matters” hereinafter), which are present on surfaces of inspection matters such as semiconductor wafers (wafers). More specifically, the present invention is directed to a foreign matter inspecting method and a foreign matter inspection apparatus, capable of judging whether or not the foreign matters are present by employing threshold values.
2. Description of the Related Art
In foreign matter inspection apparatuses capable of detecting foreign matters of semiconductor wafers, optical beams such as laser light are irradiated onto surfaces of these semiconductor wafers, and then, either reflected light or scattered light, which are generated from the surfaces of the semiconductor wafers, are detected in order that foreign matters present on the surfaces of semiconductor wafers can be detected.
In such a case where patterns which constitute respective chips have been formed on a surface of a semiconductor wafer, an image signal is formed from intensity of either reflected light or scattered light which are detected; image signals of adjoining chips are compared with each other, or the formed image signal is compared with an image signal as to a good chip which has been previously prepared; and then, when a difference between these image signals is larger than, or equal to a threshold value, it is normally so decided that the detected chip has a foreign matter.
The threshold value is calculated in such a manner that images of all of chips which are arranged along an X-coordinate direction and are detected by a sensor are overlapped with each other, and then, the threshold value is calculated from variations (standard deviation) of images at the substantially same positions within the chips. As a consequence, a judgment is made whether or not a foreign matter is present based upon a lower threshold value at a small variation, and based upon a higher threshold value at a large variation.
An adjustment of the threshold values is related to a yield of products. The yield aspect is described in, for instance, JP-A-2001-160572 and JP-A-9-74056.
Also, a foreign matter inspection apparatus for detecting a foreign matter of a wafer contains a foreign matter detecting system, and a surface detecting system. In the foreign matter detecting system, while an optical beam such as laser light is irradiated onto a surface of a semiconductor wafer, either reflected light or scattered light, which are generated from the surface of the semiconductor wafer, are detected, so that foreign matters present on the surfaces of semiconductor wafers can be detected. The surface detecting system keeps a distance of the wafer surface with respect to the foreign matter detecting system constant.
Conventionally, a distance between an objective lens of a foreign matter detecting system and a surface of a wafer has been assembled and adjusted in such a manner that this distance may becomes a focal distance based upon a designing specification of this objective lens.
Then, while this adjusted condition is continuously maintained under the same condition even in such a case where sorts of wafers to be inspected are different from each other, and manufacturing process steps of semiconductor devices are changed, the inspections have been carried out.
However, as to focal point positions of foreign matter detecting systems, these focal point positions of the foreign matter detecting systems are different from each other depending upon sorts of wafers to be inspected and manufacturing process steps of semiconductor devices. As a consequence, the conventional foreign matter inspection apparatuses have such problems that the detection performance owned by the foreign matter detecting systems cannot be sufficiently reflected, but the detection performance is different from each other, depending to the sorts of wafers and the steps.